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 1 

Proceedings of the ASME/ASCE/IEEE 2011 Joint Rail Conference 

JRC2011 

March 16-18, 2011, Pueblo, Colorado, USA 

JRC2011-56054 

DEFORMATION BEHAVIOR OF WELDED STEEL SANDWICH PANELS UNDER 

QUASI-STATIC LOADING 

Michael E. Carolan 

David Y. Jeong 

A. Benjamin Perlman 

Yim H. Tang 

Volpe National Transportation Systems Center 

Research and Innovative Technology Administration 

US Department of Transportation 

Cambridge, MA, USA 

Francisco González III 

Federal Railroad Administration 

US Department of Transportation 

Washington, DC, USA 

ABSTRACT 

For the past two decades, the Federal Railroad Administration 

(FRA) Office of Research and Development has sponsored 

research conducted by the Volpe National Transportation 

Systems Center (Volpe Center) in safety matters related to the 

transportation of hazardous materials by railroad tank cars. 

Recent research conducted by the Volpe Center has included the 

application of semi-empirical and computational (i.e., finite 

element analysis) methods to estimate the puncture resistance 

of conventional railroad tank cars under generalized head and 

shell impact scenarios. Subsequent work identified sandwich 

structures as a potential technology to improve the puncture 

resistance of the commodity-carrying tank under impact 

loading conditions. 

This paper summarizes basic research (i.e., testing and analysis) 

conducted to examine the deformation behavior of flat-welded 

steel sandwich panels under two types of quasi-static loading: 

(1) uniaxial compression; and (2) bending through an indenter. 

The objectives of these tests were to: (1) confirm the analytical 

and computational (i.e., finite element) modeling of sandwich 

structures, (2) examine the fabrication issues associated with 

such structures (e.g., material selection and welding processes), 

and (3) observe the deformation behavior and local collapse 

mechanisms under the two different types of loading. In 

addition, the uniaxial compression tests were performed to rank 

or screen different core geometries. Five core geometries were 

examined in the compression tests: pipe or tubular cores with 

outer diameters equal to 2, 3, and 5 inches; a 2-inch square 

diamond core; and a double-corrugated core called an X-core 

with a 5-inch core height. 

The compression tests showed excellent repeatability of 

structural (i.e., force-crush) response for panels with similar 

cores and welding. The 3-inch pipe core and the diamond core 

were selected as candidate cores for the next test series because 

they possess attributes of moderate strength and moderate 

relative density. In addition, force-crush curves calculated from 

finite element analysis were in reasonable agreement with the 

measured curves for all cores. 

Bend tests using a 12-inch by 12-inch indenter with 1-inch 

radius rounded edges were also conducted. The panels were 

simply-supported over 4-inch diameter rollers spanning 24 

inches between the centers of the rollers. The bend tests 

included three variables: (1) core type (diamond core and 3-

inch pipe core); (2) core orientation relative to the supports 

(cores running either parallel or perpendicular to the rollers 

used to support the panels); and (3) face sheet type (solid plates 

on both sides, strips used as face sheets on both sides, and a 

combination of solid plates and strips. Finite element analysis 

of the bend tests produced nearly identical shapes to the 

measured force-displacement curves. 

INTRODUCTION 

Previous work has identified a number of potential strategies 

for improving the crashworthiness performance of tank cars 

involved in impacts due to collisions and/or derailments [1]. 

One of the tools that can be used to implement these strategies 

is the engineered metal sandwich structure [2]. A series of tests 

was developed to better understand the behavior of sandwich 

panels under compression and bending loads, examine the 

effects of core geometries on panel deformation behavior, and 

assist in the selection of desirable panel attributes for service as 

tank car protective structures. 

As discussed in previous publications, limited space exists 

between the outside of an existing tank car and the boundary of 
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 2 

the AAR limiting outline for interchange service [1]. This 

space constraint limits the technologies that can be installed 

exterior to the tank shell for the purpose of absorbing collision 

energy. However, the tank itself possesses significant energyabsorbing capacity through plastic deformation and sloshing of 

the commodity within the tank. Puncture of the tank has been 

observed to occur around the impacting object’s perimeter in 

both tests and analyses [5]. The tank’s energy-absorbing ability 

can be taken advantage of if this localized puncture can be 

delayed during an impact event. Because a larger impacting 

object loads a larger area of tank, more energy is required to 

puncture a tank as the loaded area is increased [5, 3]. 

The sandwich panel is one technology that is capable of this 

blunting effect. When an impacting object strikes the sandwich 

panel, the panel deforms in such a way as to present the tank 

with a larger impacting object. From finite element analyses, 

the blunting advantages of the sandwich panel are seemingly 

lost when fracture of either facesheet or core initiates. The 

initiation of fracture within the panel is generally followed by 

progressive failure of the panel and the underlying tank. 

Because of the competing needs for a flexible panel that 

conforms to an impacting object but a tough panel that resists 

tearing, testing was utilized to examine the benefits and 

disadvantages of various panel designs. 

TEST 1 – UNIAXIAL COMPRESSION 

Uniaxial compression tests were performed in May of 2009 

using facilities at Lehigh University. Specifically, the 5-million 

pound capacity universal testing machine at Fritz Laboratory 

was utilized for this testing series. This test was designed to 

measure the force versus displacement behavior of sandwich 

panels when subjected to a uniform loading across their whole 

facesheets. Because this test loaded both the top and bottom 

facesheets uniformly, this test emphasized core behavior rather 

than facesheet behavior. This test series was also used to 

determine the mode shapes of the panels as they crushed, as 

well as compare manufacturing techniques used in the 

assembly of the panels. 

Panel Design

Three basic core shapes were examined in this test: round 

(tube) cores, square (diamond) cores, and X-cores. For the tube 

cores, tubes of three diameters were used for the panels: 2-inch 

outer diameter, 3-inch outer diameter, and 5-inch outer 

diameter. The square tubes measured approximately 2 inches 

on a side, for a core height of 2.6 inches with the tubes rotated 

45 degrees. The X-cores had a height of 5 inches. For all 

cores, the core thickness was approximately 0.125 inches. The 

three core shapes are shown in Figure 1. 

Figure 1. Schematic View of Three Core Geometries 

For each of the cores tested, the influence of material properties 

was desired to be minimized. For this reason, AISI 1010 was 

selected for use as the core material on the basis of its 

availability in all core configurations that were to be tested. 

Because the loads were being applied across the entire surface 

of the facesheet, the selection of facesheet material was not 

critical to the success of this test. ST100XF Domex steel was 

selected for the facesheets. This material is a high-yield steel 

that is typically used in structural applications. 

Stress-strain and elongation data were measured for each of the 

materials used in this test. Five samples of each material were 

tested. The average results for each material are reported in 

Table 1. 

Table 1. Material Properties 

Material 

Yield 

Stress 

(ksi) 

Ultimate Stress 

(ksi) % Elongation 

Domex Plate 101.2 113.2 17.8 

2 inch diamond 1010 60.5 71.4 21.9 

2 inch round 1010 52.3 62.6 32.2 

3 inch round 1010 43.0 53.6 35.5 

5 inch round 1010 37.1 51.8 38.7 

This material data was used in the finite element modeling for 

both the compression and bending test series. Finite element 

analysis was performed using Abaqus/Explicit, a commercial, 

general-purpose finite element analysis software package [6]. 

Non-linear elastic-plastic material behavior was modeled using 

an isotropic hardening behavior to describe the materials once 

yield had been exceeded. 

A total of fourteen panels were fabricated for the first series of 

tests. The panels were all six inches deep by 24 – 27 inches in 

width. The width varied based upon the number of cores in the 

particular panel. Core-to-facesheet welding was performed 

either manually or robotically for each panel. Duplicate panels 

of the same geometry were fabricated either with manual 

welding or robotic welding to allow for comparison of the two 

fabrication techniques. A description of the test articles is 

provided in Table 2. 
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 3 

Table 2. Test Articles for Compression Test

Test Setup and Results

Panels were placed on a flat block in the universal testing 

machine. The movable test head was positioned slightly above 

the support block at the top of the test article and progressed 

downward at a target speed of 0.2 inches per minute. This 

speed was chosen as sufficiently slow to prevent the 

introduction of dynamic effects into the test results. 

Figure 2. Schematic Test Setup for Crush Test 

Displacements were measured via two string potentiometers 

mounted between the fixed and movable heads of the universal 

testing machine. Applied load was recorded via load cell in the 

movable head of the machine. Video cameras were positioned 

to record two views of the crush tests as they progressed. 

Characteristic Deformation Modes

Each core geometry exhibited a unique load-displacement 

characteristic and crushing mode shape. For each of the three 

core geometries examined, the load-displacement results were 

fairly repeatable from test article to test article. Additionally, 

the characteristic crushing behavior was similar for pipe cores 

of the same diameter, and for the diamond cores. These results 

are discussed below for the three categories of core geometries: 

X-core, diamond core, and pipe core. 

For the X-core geometry, a total of four test articles were 

constructed and tested. The geometry was the same for each 

sample, but different methods of welding were used to 

assemble each panel. Each panel was approximately 5” high. 

As can be seen in Figure 3, the characteristic load-displacement 

behavior is similar for all panels. The X-cores begin to crush 

with a relatively high force that peaks after approximately 1⁄2” 

of crush. The load then drops as plastic hinges begin to form in 

the legs of the cores. As the legs of the cores deform 

significantly, they begin to contact one another and the force 

required to further crush begins to increase. Eventually, the 

core material has experienced sufficient solidification to cause 

the load to increase nearly vertically with small increases in 

displacement. 

0

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80

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120

01234

Displacement (in)

Load (kips)

X-core, Spot Weld (0047-09)

X-core, Spot Weld (0047-10)

X-core, Slot & Spot Welds (0047-11)

X-core, Slot and Spot Welds (0047-12)

Figure 3. Load-displacement Results for X-cores, Crush Test 

The crush tests performed on the X-cores exhibited the least 

repeatability in terms of mode shape of the crushed sample. As 

seen in Figure 4, each X-core within a given sandwich panel 

demonstrated a unique failure mode. Additionally, a number of 

X-core panels experienced failed spot welds either between 

cores or between the core and the facesheet. 

Panel 

No. 

Core 

Material 

Weight 

(lbs) Fabrication Method 

0047-01 5" round pipe 36.5 Slot/skip (manual) 

0047-02 5" round pipe 36.4 Slot/skip (robotic) 

0047-03 3" round pipe 33.9 Slot/skip (manual) 

0047-04 3" round pipe 33.8 Slot/skip (robotic) 

0047-05 2" round pipe 34.3 Slot/skip (manual) 

0047-06 2" round pipe 34.3 Slot/skip (robotic) 

0047-07 2" round pipe 34.2 Slot/skip, stitch between cores 

(manual) 

0047-08 2" round pipe 34.3 Slot/skip, stitch between cores 

(robotic) 

0047-09 5” X-core 40 Spot (manual) 

0047-10 5” X-core 40 Spot (manual) 

0047-11 5” X-core 40 Slot & spot (manual) 

0047-12 5” X-core 40 Slot & spot (robotic) 

0047-13 2" square 

pipe 33.1 Slot/skip (robotic) 

0047-14 2" square 

pipe 33.4 Slot/skip, stitch between cores 

(robotic) 
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 4 

Figure 4. Progressive Crush of X-core Sample 

Two sandwich panels were constructed using 2” square tubes 

rotated 45 degrees to form diamond-shaped cores. In one of 

these two panels, adjacent diamonds were stitch-welded to one 

another at the 3- and 9- o’clock positions. For both diamond 

panels, the load-displacement results are plotted in Figure 5. 

Both panels exhibited an initially high peak force of nearly the 

same magnitude. 

0

100

200

300

400

500

600

700

800

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Displacement (in)

Load (kips)

Diamond Core (0047-13) Diamond Core w/stitch (0047-14)

Figure 5. Load-displacement Results for Diamond Cores, Crush 

Test 

As each core was compressed vertically, it attempted to expand 

laterally. However, the adjacent cores were also trying to 

expand laterally, causing the cores to “lock” together. 

Eventually, plastic deformation would occur in one or more 

legs of the cores, and the force level would drop. The force 

level would remain low until the cores were sufficiently 

compacted to form an effectively solid core to the sandwich 

panel, and the force would increase dramatically. Progressive 

crush of a diamond core sample is shown in Figure 6 

Figure 6. Progressive Crush on Diamond Core Sample 

A total of eight panels were constructed using round cores. 

There were three sizes of core used: 2” diameter, 3” diameter, 

and 5” diameter. Two samples of each size were fabricated. 

Two additional panels were constructed using 2” pipes that 

were then stitch welded together at their 3- and 9-o’clock 

positions. The load-displacement characteristics for all eight 

round core panels are shown in Figure 7. 

0

100

200

300

400

500

600

700

01234 Displacement (in)

Load (kips)

5" Round Pipe (0047-01)

5" Round Pipe (0047-02)

3" Round Pipe (0047-03)

3" Round Pipe (0047-04)

2" Round Pipe (0047-05)

2" Round Pipe (0047-06)

2" Round Pipe w/Stitch (0047-07)

2" Round Pipe w/Stitch (0047-08)

Figure 7. Load-displacement Results for Tube Cores, Crush Test 

The characteristic crushing behavior of the round cores is 

different from either the X-core or the diamond core panels. 

Where those configurations featured an initial high peak force 

the round cores all exhibit a rise in force followed by a 

gradually-increasing force plateau. Following this plateau, the 

cores exhibit a steeper rise in force and a peak crushing force. 

Once overcoming the peak the force drops, until the core has 

solidified. At that point, the force increases dramatically, as the 

core is nearly a solid at this stage. A series of photographs from 

the crush test of one of the 3” diameter pipe core panels is 

shown in Figure 8 
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Figure 8. Progressive Crush on Tube Core Sample 

The load-displacement plots in Figure 7 indicate three distinct 

groups of data. The lowest two load-displacement plots 

correspond to the 5” tubes, the middle two series are the data 

from the 3” diameter tubes, and the highest series correspond to 

the 2” diameter tubes. As the diameter of the tube decreases, 

the force levels to crush the tubes increase. 

Regardless of the diameter of the tubes, the mechanism by 

which the tubes fail is the same. As the load is applied, the tube 

is squeezed at the top and the bottom. Each tube attempts to 

deform outward at the 3- and 9-o’clock positions, but is met 

with resistance from the adjacent tube. Because the panels 

were constructed with the tubes in close proximity to one 

another, the tubes contact one another nearly immediately once 

crushing has begun. The tubes then begin to square off, 

forming 8 plastic hinges around their circumference. These 

plastic hinges were predicted for tubes with internal bracing; 

the presence of adjacent tubes restricts the mode of deformation 

in much the same way as internal bracing would [4]. Because 

the larger diameter tubes have a larger moment arm between 

the applied loads and the hinge locations, a smaller force is 

required to initiate plastic hinge formation, given that all tubes 

used 0.125” steel of the same type. 

During crush of the 2” diameter tubes, large lateral deflections 

were observed in all test samples. Because the tube wall 

thickness was maintained at 0.125” for all diameters of tubes, 

this resulted in a higher core density as the diameter of the tube 

was decreased. From Figure 7, the loads required to crush the 

2” cores were much higher than the loads required to crush any 

other core examined. For all other cores, friction between the 

support blocks in the testing machine and the facesheets was 

sufficient to prevent lateral motion of the test article. However, 

due to the high applied loads and the formation of plastic 

hinges, the 2” tube core sandwich panel was able to overcome 

friction and shift laterally. This behavior was observed in all 

four 2” sandwich panels, and is shown in Figure 9. 

Figure 9. Deformed Shape of 2" Tube Core Showing Relative 

Motion of Facesheets 

Finite Element Analysis Results

Overall, the pre-test finite element analyses predicted modes of 

deformation of the panels that were consistent with what was 

seen in the testing. Additionally, the pre-test finite element 

models generated load-displacement characteristics that were 

qualitatively similar to the results recorded in the test. 

However, finite element analysis results nearly universally 

over-predicted the force levels experienced by the panels in the 

test. Figure 10 shows test results for 5” pipe panels and the 

finite element model of the panels. As this figure shows, the 

finite element results follow the same trend as both sets of test 

data, with peaks occurring at approximately the same levels of 

displacement. However, the finite element model over-predicts 

the force necessary to crush the panel at all levels of 

displacement. 

0

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20

30

40

50

60

70

01234

Displacement (in)

Load (kips)

5'' Round Pipe (0047-01) 5'' Round Pipe (0047-02) 5" FEA

Figure 10. Example Force-displacement Results from Crush Test 

This tendency to capture the qualitative behavior of the panels 

while over-predicting the actual force level may be attributed to 

the perfect geometry and lack of residual stresses incorporated 

in the FE model. In reality, the tubes used to make up the cores 

of the sandwich panels retain some residual stresses from their 

manufacture, as well as small imperfections in material. This 

allows collapse of the tubes to occur at a lower load than 

predicted by the FE model. 

Figure 11 shows a comparison between the deformed shape at 

an intermediate point during the crushing of a 3” round core 

sample. The top half of this figure contains a photograph of the 

actual test article, while the bottom half contains an image from 

a finite element analysis. The two images demonstrate that the 

FEA is adequately capturing the deformation mode of the 

sandwich panel for this load case. In general, the FE model 

results accurately described the mode shape experienced by the 

corresponding panel during testing. 
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 6 

Figure 11. Comparison Between Deformed Shapes in Test and 

Analysis for 3” Round Core 

TEST 2 – 3 POINT BENDING 

The second test to be performed in this series was a 3-point 

bending test of a group of sandwich panels. This test series was 

designed to examine loading where the anticipated failure mode 

would result in large deformation of the facesheets, not solely 

crush of the cores. Based partially on the results of the first 

test, the field of possible cores was narrowed. The X-core 

geometry was eliminated from this test series due to the overall 

poor performance of the welds of the X-core in the first test, as 

well as the relatively low crush force offered by the X cores 

once the initial peak was overcome. The 5-inch pipes were 

eliminated due to their low crush force as well. The 2-inch 

pipes were eliminated from further testing due to the amount of 

welding necessary to assemble a panel made up of such small 

pipes. Additionally, because of the difficult-to-predict lateral 

shift in the 2-inch pipe panels these panels were eliminated. 

The remaining configurations that were tested in the second test 

were a 3-inch outer diameter round tube and a 2-inch square 

diamond. These two cores were selected based on their 

performance in the first test as well as their relative ease of 

manufacture A total of 16 panels were built and tested in 

bending. The panels all measured approximately 48” in width 

by 18” in depth. The test articles are described in Table 3. 

Table 3. Test Articles for Bending Test 

Panel 

No. Core Type Core 

Orientation Facesheet Type Weight 

(lbs) 

0047-15 Diamond Parallel Solid 139.5 

0047-16 Diamond Parallel Solid 139.5 

0044-17 Diamond Perpendicular Solid 140 

0047-18 Diamond Perpendicular Solid 137 

0047-19 3-inch Pipe Parallel Solid 139.5 

0047-20 3-inch Pipe Parallel Solid 139.5 

0047-21 3-inch Pipe Perpendicular Solid 141 

0047-22 3-inch Pipe Perpendicular Solid 142.5 

0047-23 Diamond Perpendicular Strip 109.5 

0047-24 Diamond Parallel Strip 111.5 

0047-25 3-inch Pipe Perpendicular Strip 110.5 

0047-26 3-inch Pipe Parallel Strip 111 

0047-27 Diamond Perpendicular Strip 137 

0047-28 Diamond Parallel Strip 109 

0047-29 Diamond Perpendicular Solid & Strip 126 

0047-30 Diamond Perpendicular Solid & Strip 126 

A simple 3-point bending test setup was used for this test. As 

shown schematically in Figure 12, the sandwich panel was 

supported by two 4-inch diameter round steel bars placed 24 

inches apart. These bars were welded to a thick steel plate, 

which was placed on the fixed head of the universal testing 

machine. Load was applied through a 12-inch x 12-inch load 

head with 1-inch radius rounded corners. This load head was 

mounted on the movable head of the universal testing machine. 

The load head was centered on the sandwich panel in both the 

width and depth dimensions. 

Figure 12. Schematic Test Setup for Bending Test 

Test articles were constructed with cores oriented either 

perpendicular or parallel to the direction of the supports. This 

was done to examine the performance of the different cores 

with respect to orientation on a tank, which exhibits anisotropic 

bending stiffness along its length versus around its 

circumference. 

For the bending test, core thickness was maintained at 

approximately 0.125 inches for the diamonds and for the tubes. 
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 7 

Both cores were made of AISI 1010 steel. The facesheets were 

made of 0.125-inch Domex steel. The thinner facesheets were 

chosen to save weight in the panels as well as to decrease the 

likelihood of localized tearing of the facesheets due to 

facesheets that were overly stiff. 

One concern with the sandwich panel approach to preventing 

tank car puncture is the possibility of a localized tear in the 

facesheet spreading along the perimeter of the impacting object. 

If this occurs, the impacting object may locally tear through the 

sandwich structure without engaging the structure outside of 

the impact area. This ability to “short circuit” the benefits of 

the sandwich panel on tank car protection is highly undesirable. 

As a measure for preventing the propagation of cracks in the 

facesheets, the use of strip facesheets was suggested for this 

test. Rather than using monolithic facesheets, as are typically 

used in sandwich panel construction, strips of material are used 

as facesheets on one or both sides of the sandwich panel. In the 

event of a crack forming in a facesheet, the crack is effectively 

arrested when it reaches the edge of a strip and cannot jump to 

another strip. In a monolithic facesheet, the crack could 

continue traveling across the width of the facesheet. Solid and 

strip facesheets are shown in Figure 13 for two sandwich panels 

with 3” tubes in parallel orientation. For any panel constructed 

using strip facesheets the pattern chosen was a 3-inch strip of 

material adjacent to a 3-inch gap with no facesheet. 

Figure 13. Pipe Core Sandwich Panel with Solid (top photo) and 

Strip (bottom photo) Facesheets 

Bending Behavior of Panels

For all panels, the behavior under three point bending load can 

be divided into three general phases. These phases are shown 

schematically in Figure 14. In the first phase, the panel 

undergoes elastic bending. In the next phase, the panel is 

experiencing a localized buckling. This phase is characterized 

by a plateau or slight slope to the applied force. In the third 

phase, the bottom facesheet has come into contact with the 

fixed load head and the cores are experiencing crushing. The 

crushing behavior of the cores beneath the load head is similar 

to the behavior of the corresponding core configuration tested 

in uniaxial compression in Test 1. 

Figure 14. Schematic Force-displacement Curve for Bending Test 

Core Orientation

For both the pipe core and the diamond core panels, trends 

emerged in the behavior of the panels during the bend testing. 

Regardless of the core geometry, panels built with cores 

oriented perpendicular to the supports required a larger force to 

deform than a panel with the same core oriented parallel to the 

supports. For the panels with cores oriented perpendicular, the 

entire panel resists bending from the first contact of the rigid 

block. For panels with the cores parallel to the supports, the 

cores are free to move past one another, and only the facesheets 

provide the initial bending strength. Example loaddisplacement characteristics are shown in Figure 15 for pipe 

core panels with strip facesheets. The results from cores 

oriented parallel to the supports are plotted alongside results 

from cores perpendicular to the supports. The perpendicular 

cores require more force to bend than the parallel cores. Finite 

element analysis results are plotted for both core orientations 

and show good agreement with the test data. 

Pipe Cores with Strip Facesheets - Orientation Compared

0

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150

200

250

300

350

400

01234567

Displacement (inches)

Load (kips)

Perpendicular, Strip (0047-25)

FEA - Perpendicular, Strip

Parallel, Strip (0047-26)

FEA - Parallel, Strip

Figure 15. Load-displacement Behavior for Pipe Cores with 

Different Core Orientations 

As seen in Figure 15, finite element analysis results compare 

favorably with the test results for each compared panel. 
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 8 

Because of the bending test setup, during the first four inches of 

displacement the panel is in bending. After that displacement 

the bottom facesheet has contacted the supporting structure and 

crush of the cores occurs. Finite element results compare more 

favorably in the bending phase of the test than in the 

compression phase, where the analysis tends to over-predict the 

response. Based on the results of the bending and crush tests 

and the corresponding analyses, it appears that crush of the 

cores is more sensitive to small imperfections and residual 

stresses than bending of the panels. 

Pipe and Diamond Core Behavior

Panels with at least one strip facesheet were constructed using 

pipe core and using diamond cores. Regardless of the core 

geometry, panels with solid facesheets were stiffer than panels 

with strip facesheets. The solid facesheet offers more 

resistance to bending than the strip facesheets, which cannot 

transfer bending loads from one strip to another. 

Figure 16 is a load-displacement characteristic plot for 

perpendicular-oriented panels with solid facesheets. This figure 

contains data from diamond and pipe core tests. These results 

are selected for discussion because the load cases with solid 

facesheets and perpendicularly-oriented cores are the stiffest 

panels relative to any other configuration tested. The diamondcore panels had a more uniform bending strength than the pipe 

cores, which reached a local maximum and dropped off before 

contacting the bottom load head. 

 

Perpendicular Orientation, Solid Facesheets

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0123456

Displacement (inches)

Load (kips)

Diamond Core (0047-17)

Diamond Core (0047-18)

Pipe Core (0047-21)

Pipe Core (0047-22)

Figure 16. Load-displacement Characteristics for Panels with 

Perpendicular Orientation and Solid Facesheets 

During the test, localized crushing of the cores was observed on 

the pipe-core panels at three locations: beneath the load head 

and above each of the two supports. During the tests of the 

diamond-core panels, no such localized crush was observed at 

the supports. Figure 17 indicates the deformation in the top 

photo of the pipe core sample alongside a diamond core photo 

where no such deformations are observable. 

Figure 17. Localized Indentation in Pipe Core (top) and Absence 

of Indentation in Diamond Core (bottom) 

During a bending test of a sandwich panel, competing modes of 

deformation are possible for panels of different designs. 

Results from the bending test are compared to results for the 

crush test of the diamond cores in Figure 18. In this plot, the 

initial peak crush strength of the diamond cores far exceeds the 

initial bending load of the diamond panel. Because of this, the 

panel is more likely to experience gross bending until 

contacting the bottom load head, at which point crushing will 

occur. 

0

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300

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500

0123456

Displacement (inches)

Load (kips)

Diamond Crush (0047-13) Diamond Crush (0047-14)

Diamond Bending (0047-17) Diamond Bending (0047-18)

Figure 18. Diamond Core Load-displacement Behavior from 

Compression Test and Bending Test 

A similar plot is shown for the 3-inch diameter tube cores in 

Figure 19. Because the tubes do not have an initial high peak 

crush load, the deformation mode is less clear-cut. In the crush 

test, the pipe cores experience a relatively steady crushing load 

as the result of plastic hinge formation and spreading. From the 

results of the bending test, the panel appears to simultaneously 

experience crush of the cores under the load head and the two 

supports as well as gross bending of the panel. 
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 9 

0

100

200

300

400

500

0123456

Displacement (inches)

Load (kips)

Pipe Crush (0047-03)

Pipe Crush (0047-04)

Pipe Bending (0047-21)

Pipe Bending (0047-22)

Figure 19. Pipe Core Load-displacement Behavior from 

Compression Test and Bending Test 

Solid Facesheets and Strip Facesheets

Figure 20 shows the load-displacement characteristics for four 

tests and two analyses of panels with parallel-oriented diamond 

cores. Two panels tested featured solid facesheets and two 

panels featured strip facesheets. For either facesheet 

configuration, the two panels demonstrated similar loaddisplacement behavior. Additionally, the finite element 

predictions matched the overall shape of the test results and 

closely matched the peak crush force for both panel 

configurations. 

Diamond Core - Parallel Orientation

0

50

100

150

200

250

300

0123456

Displacement (inches)

Load (kips)

Diamond Core, Solid Facesheets (0047-15)

Diamond Core, Solid Facesheets (0047-16)

Diamond Core, Strip Facesheets (0047-24)

Diamond Core, Strip Facesheets (0047-28)

FEA - Solid Facesheets

FEA - Strip Facesheets

Figure 20. Load-displacement Results for Parallel Diamond Cores 

The panels with strip facesheets resulted in lower forces to 

deform the panel a given distance than the solid facesheets. 

This is due to the loss of bending stiffness associated with the 

discontinuous facesheets on the strip panels. The crush strength 

of the cores is slightly reduced in the panels with the strip 

facesheets, likely due to the increased ability for the cores to 

deform without contacting facesheets in the gap spaces. 

One of the reasons for selection of strip facesheets as 

candidates for tank protection was the ability of the strips to 

prevent cracks from propagating through the facesheet and 

causing localized fracture of the sandwich panel. During 

testing, crack propagation was observed in two diamond-core 

panels, 0047-17 and 0047-18. Both of these panels featured 

cores oriented perpendicular to the supports, which results in a 

stiffer panel than cores oriented parallel to the supports. In both 

panels, cracks formed in the inner facesheet in the area around 

the welds in the center of the panel. These cracks are indicated 

in Figure 21. 

 

Figure 21. Cracks in Inner Facesheet of 0047-18 (left) and 0047-17 

(right) 

Four additional panels with perpendicularly-oriented diamond 

cores were tested in bending. Two of these panels featured strip 

facesheets on both the top and bottom, and two of these panels 

featured a solid facesheet on one side and a strip facesheet on 

the other. Of the two “hybrid” panels with one solid and one 

strip facesheet, one was tested with the solid facesheet facing 

up, and one was tested with the strip facesheet facing up. 

Load-displacement characteristics are plotted for all six 

sandwich panels with perpendicular-oriented diamond cores in 

Figure 22 

Diamond Core - Perpendicular Orientation

0

50

100

150

200

250

300

0 0.5 1 1.5 2 2.5 3 3.5 4

Displacement (inches)

Load (kips)

Diamond Core - Solid Facesheets (0047-18) Diamond Core - Solid Facesheets (0047-17)

Diamond Core - Strip Facesheets (0047-23) Diamond Core - Strip Facesheets (0047-27)

Diamond Core - Hybrid A (0047-29) Diamond Core - Hybrid B (0047-30)

Diamond Core - Solid Facesheets (FEA) Diamond Core - Strip Facesheets (FEA)

Figure 22. Load-displacement Behavior for Perpendicularlyorientation Diamond Core Panels 

Of the diamond core panels with perpendicular orientation, 

only the two panels with both solid facesheets experienced the 

crack propagation shown in Figure 21. For all of the additional 

diamond core panels with at least one strip facesheet the 

facesheets did not crack. This gives an indication that the strip 

facesheets allow greater flexibility in the deformation of the 
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.  Approved for public release; distribution is unlimited.

 10 

panel, preventing the large tensile forces that caused tearing of 

the facesheets. 

Measurement of Relative Contact Area

One of the positive attributes associated with sandwich panels 

is the ability to blunt an impact load, making the impacting 

object appear to have a larger footprint when it comes into 

contact with the supporting structure. In the case of a tank car, 

this supporting structure is the commodity tank. For a given 

impact speed and impact mass, the amount of energy needed to 

puncture a tank of given thickness increases with the size of the 

impacting object [5]. 

As a part of this series of bending tests it was desirable to 

attempt to quantify the amount of blunting, or load spreading, 

that the sandwich panels affected on the impactor. A low-cost 

method for measuring the contact area on the fixed platen was 

implemented for a small number of bending tests. A sheet of 

commercially-available bubble wrap was placed on the fixed 

platen in the area between the two supports, as shown in Figure 

23. When the inner facesheet of the sandwich panel began to 

contact the bubble wrap, the increasing force level would pop 

the bubbles. After the test was completed, the popped bubbles 

could be examined and the contact area could be determined. 

Figure 23. Bending Test Setup with Bubble Wrap to Indicate 

Contact 

The bubble wrap method was used in two test cases. In one, a 

panel with parallel-oriented pipe cores and solid facesheets had 

an inner platen contact area of 16 inches in the direction of the 

supports by 14.5 inches wide. In the second test, a panel with 

parallel-oriented diamond cores and solid facesheets had an 

inner platen contact area of 16 inches in the direction of the 

supports by 13.25 inches wide. The particular bubble wrap 

used in this test had a total sheet width of 16 inches, limiting 

the measurable range in the direction parallel to the supports. 

In both cases, the sandwich panel demonstrated its ability to 

increase the loaded area of the impactor alone. Because of the 

rounded corners of the impactor, the area initially in contact 

with the outer facesheet is 10 inches by 10 inches. During the 

deformation of the panel, the full 12 inch by 12 inch impactor 

contacts the outer facesheet. Using the 12 inch by 12 inch 

dimension as the baseline contact area, the area of contact 

reported by the bubble wrap for both tests is approximately 50 

percent greater. 

CONCLUDING DISCUSSION 

Compression tests were conducted on five sandwich panel 

geometries: 2-inch, 3-inch, and 5-inch pipe cores, diamond 

cores, and X-cores. Based on the results of these compression 

tests, 3-inch pipe cores and diamond cores were selected for 

additional examination in the form of 3-point bending tests. 

Bending tests were conducted to examine the effect of the two 

core geometries, as well as the influence of core orientation on 

bending behavior, the effectiveness of strip facesheets at 

inhibiting facesheet tearing, and the load-spreading benefits of 

the panels. 

The purpose of investigating sandwich panels is to develop 

effective strategies for enhancing the impact resistance of tank 

cars carrying hazardous materials. It is envisioned that the 

sandwich panels would be utilized as a protective outer 

covering for the tank car, in place of the conventional jacket. 

Further studies are ongoing investigating the behavior of steel 

panels used as protective structures surrounding commodity 

tanks. 

ACKNOWLEDGMENTS 

The work described in this paper was sponsored by the Federal 

Railroad Administration, Office of Research and Development. 

Mr. Kevin Kesler is the Chief of the Equipment and Operating 

Practices Division. 

Cellular Materials International (CMI) constructed the panels 

used in both tests. Dr. Yellapu Murty, Mr. Bob Kurtz, and Mr. 

Kerry Elzey have been heavily involved in discussing and 

implementing the design and fabrication of the steel panels 

used in this research effort. 

REFERENCES 

 

1. Tyrell, D.C., Jacobsen, K., Talamini, B., Carolan, M., 

"Developing Strategies for Maintaining Tank Car Integrity 

During Train Accidents," Proceedings of the 2007 ASME Rail 

Transportation Division Fall Technical Conference, 

RTDF2007-46015, September 2007. 

2. Carolan, M., Talamini, B., Tyrell, D.C., (2008). “Update on 

Ongoing Tank Car Crashworthiness Research: Predicted 

Performance and Fabrication Approach,” Proceedings of the 

2008 IEEE/ASME Joint Rail Conference, JRC2008-63052. 

 

3. Tang, Y.H., Yu, H., Gordon, J.E., Priante, M., Jeong, D.Y., 

Tyrell, D.C., Perlman, A.B., "Analysis of Full-Scale Tank Car 

Shell Impact Tests," Proceedings of the 2007 ASME Rail 

Transportation Division Fall Technical Conference, 

RTDF2007-46010, September 2007. 
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.  Approved for public release; distribution is unlimited.

 11 

 

4. She, M., and Carney, J. “Energy Dissipation Properties of 

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